1. Field of the Invention
The present invention relates to a driving apparatus that drives a brushless motor in a sensorless manner while performing switching between rectangular wave drive and sign wave drive, and to a driving method therefor.
2. Description of Related Art
As a driving method for a brushless motor, in addition to a sign wave drive, a rectangular wave drive is known. The rectangular wave drive is a driving method of determining, among three phases of a brushless motor, a phase for carrying out PWM control, a phase for outputting high signal and a phase for outputting a low signal.
Furthermore, for example, Japanese Laid-open (Kokai) Patent Application Publication No. 2009-189176 discloses a method for achieving these driving methods without employing a sensor for detecting the angle of a rotor.
In such a sign wave drive, it is possible to obtain angle information of the rotor from an induced voltage produced by rotation of the rotor (in other words, speed-induced voltage). Furthermore, in such a rectangular wave drive, it is possible to obtain angle information of the rotor from a voltage of a non-energized phase induced by application of pulse-shaped voltage to an energized phase (in other words, transformer-induced voltage).
In a sensorless driving apparatus for a brushless motor, there is a case in which drive mode is switched from rectangular wave drive to sign wave drive according to rotation speed of the motor. Here, when the rotation speed of the motor reaches a speed for switching the drive mode within a switching cycle of energizing mode in rectangular wave drive, switching to sign wave drive is performed in a state that the angle of the rotor is unknown.
In the sign wave drive, since position information in shorter angular cycle as compared with a rectangular wave drive is required, if the drive mode is switched to sign wave drive in a state that the angle of rotor is unknown, controllability immediately after the switching is deteriorated, so that loss of synchronism or drop of torque occurs.